The present invention relates to a method for producing, maturing and drying negative and positive plates for lead accumulators, initially, in a pasting step, the plates being produced by bringing in the lead paste, consisting of the main components of lead oxide, water and sulfuric acid, as active composition in an electrode support.
According to the prior art, known from the relevant practice, lead accumulators are produced by producing the active compositions for the negative and positive plates from the main components lead oxide, water and sulfuric acid. These materials are processed in a mixing process into a pasty lead paste. Within the mixing process, additives, which are usually referred to as spreading agents and are essentially barium sulfate, carbon black as well as special forms of lignin compounds and/or humic acid, are also added to the negatively active compositions. The negative and positive plates are produced by bringing the negative, active composition or the positive active composition into an electrode support. The introduction of the pasty lead paste into the electrode support is referred to as pasting and usually is accomplished by machine on appropriate pasting lines, which consist, essentially, of a pasting machine and a downstream pre-dryer.
The pre-dryer superficially dries the plates, so that tack-free and, with that, stackable plates are obtained. When stacking the plates, a residual moisture content of the active composition of less than 9% by weight is usually aimed for, in order to avoid adhesion of the plates.
For continuously produced electrode supports, release paper, which permits an increase in the residual moisture up to approximately 12% by weight, is applied on the upper and lower side of the plates produced.
Usually, gravity casting lattices or metal mesh lattices or continuously cast or stamped lattices are used as electrode supports.
The plates usually are stacked prone at the end of the pasting line predominantly fully automatically, partially also manually and are deposited vertically or predominantly horizontally on pallets. In the case of gravity casting lattices, double plates are partly also suspended loosely in frames.
The active compositions are converted in a subsequent step of the process, the so-called maturing and drying step, into a porous, cross linked structure, preferably of tribasic and/or tetrabasic lead sulfate crystals and the active composition is tied to the electrode support by an oxidation of the surface of the electrode support. By means of the quality of the cross linking of the tribasic and/or tetrabasic lead sulfate crystals, by the extent of the porosity of the active composition, as well as by the tying to the electrode support, this step of the process essentially determines the electrical performance data and the service life of the lead accumulator.
According to the prior art, the maturing and drying takes place almost exclusively in batch chambers, which provide temperature and humidity control during the maturing phase and drying during the drying phase.
Dusty lead oxides, containing an appreciable amount of unoxidized lead, are used for the production of lead accumulators. The lead oxides used usually contain 25% to 35% by weight of unoxidized lead. During the maturing, there is an exothermic oxidation of the residual lead of the lead oxides used. The exothermic reaction depends essentially on the method, by which the lead dust is produced. As a rule of thumb, lead oxides, produced by the Barton method, tend to oxidize spontaneously less than do lead oxides from lead mills. There are also appreciable differences between the plants of different manufacturers with respect to the rate of spontaneous oxidation of the lead oxides.
The spontaneous oxidation of the lead oxides within the active compositions in the finished electrode supports always takes place preferably at a moisture content of 5 to 7% by weight of the active composition. The active composition accordingly tends to oxidize during the maturing. For its part, the exothermic reaction during the oxidation increases the drying of the active composition. Therefore, during the maturing, the charging time of the batch chambers, as well as the total maturing time for forming a tribasic and/or tetrabasic lead sulfate crystalline framework should therefore be kept a short as possible, in order to avoid any drying out, because this drying out would counteract the formation of a stable crystalline framework as well as a good chemical bonding to the electrode support.
The usual batch chambers for maturing and drying plates for lead accumulators have the capacity to accommodate several hours, usually 8 to 16 hours, of production. Negative and positive plates usually are matured within 16 to 24 hours to tribasic lead sulfate crystals and subsequently dried for 1 to 3 days.
The maturing to tetrabasic lead sulfate crystals has increasingly gained in importance with the introduction of electrode supports based on calcium alloys for positive plates. Tetrabasic crystalline structures occur only at temperatures above about 70° C., are formed at a relatively high humidity within about 2 to 6 hours, but have very large crystals, which have a negative effect on the internal surface area of the porous electrodes and require a distinctly longer formation time of the electrodes with an increased requirement for electrical energy. Advantageously, however, positive plates with tetrabasic lead sulfate crystals have an improved service life as well as protection against passivation in the case of an excessive discharge (antimony-free effect).
For the production of active compositions for positive plates, the size of the tetrabasic crystals can be controlled by the addition of micronized tetrabasic lead sulfate. Crystals, comparable in size to those of tribasic lead sulfates, can be produced. The concept of micronized tetrabasic lead sulfate is understood to refer to crystal sizes of less than 1 μm of comminuted tetrabasic lead sulfate with the addition of finely divided pyrogenic silica, as described in the document WO2004/059772 A2. The positive, active composition is added during the production of the tetrabasic micronized lead sulfate. During the subsequent maturing at elevated temperatures, preferably in an atmosphere saturated with water vapor, a complete small tetrabasic crystalline structure is formed already within one hour. This also is described in the document WO2004/059772 A2.
According to the prior art, the rapid maturing of plates for lead accumulators in the case of freely exposed surfaces is known. In the document EP 0 949 700 B1, a method is described, in which lead plates are matured and dried within a few hours in a continuous 3-step process. It is described that the moisture content of the plates is controlled by means of the use of a humidity-controlling membrane and by means of a plate surface, which is kept free for a uniform treatment. Due to the exposed surfaces, the metal of the residual lead is broken down and rapid drying can take place. It is an important prerequisite of the method that both surfaces of the plate are kept free for a uniform treatment and are provided with a humidity-controlling membrane, which can transport and store moisture. The use of the separator, which is a component of lead accumulators, as the humidity-controlling membrane, is proposed as a particularly advantageous solution.
The EP 1 235 287 A1 also shows a method for maturing positive lead accumulator plates, for which it is important to isolate the plates. In a further development of claim 4 of this publication, the plates are isolated by separating them by means of a humidity-controlling membrane. By these means, it is intended to achieve that a water vapor treatment for maturing the plates has to be carried out only for a few hours. This is attributed especially to the fact that the plates for the maturing process are isolated. The isolation should take place at least during the treatment with water vapor.
For the usual manufacturing technology described above, it is not necessary to keep the two plate surfaces free or provide them with a humidity-controlling membrane. Rather, the usual practice is to stack the plates after they are pasted without any humidity-controlling membrane and without a space between the plates. For this reason, the technique of maturing plates rapidly of the EP 0 949 700 B1 or of the EP 1 235 287 A1 cannot be used without major changes with respect to the plant technology. The high investment costs and the complex technical realization for introducing the humidity-controlling membranes oppose the spread of the technology described in the EP 0 949 700 B1 and the EP 1 235 287 A1.
One possibility for keeping the surfaces of plates, stacked in stacks, free without introducing humidity-controlling membranes, consists of using supporting racks or frames, in which the plates are disposed with essentially a vertical alignment. At the same time, in these carrier racks or frames, due to the movement clearance required for loading and unloading, the stacks of plates experience a loosening, which to some extent produces exposed surfaces with very small gaps. For this purpose, the plates must be placed upright, in order to achieve the certain loosening of the plates, which thereafter are standing on the carrier rack or on the frame. The dimensions of the stack of plates must therefore be less than the clear width of the carrier rack or frame. Plate stacks, in which the plates are stacked horizontally, cannot be used here. It is a disadvantage of this method that it is necessary to use carrier racks or frames, which are matched to the special geometry of the plates. Changing the plate geometry also necessarily requires a change in the carrier rack or frame or the use and exchange of exchangeable inserts in the carrier racks or frames.
It is therefore an objective to provide a method of the type named above, which avoids the disadvantages of the prior art, which have been explained above, and with which plates with improved properties can be produced with great economic efficiency.